This invention relates to the field of lasers. Many applications require the generation of extremely short, high-peak-power pulses of light from a laser. (For the purpose of this discussion, extremely short will refer to pulse durations of about 1 ns or less; high peak power will refer to peak powers of about 10 kW or greater.) One method for producing extremely short pulses is to mode lock the laser. In mode locking, several longitudinal modes of a laser are locked together such that a periodic train of extremely short pulses is produced. The period between pulses is the round-trip time of light in the laser cavity, typically 10 ns. Because of the large number of pulses produced each second, even lasers with high average power (10 Watts-100 Watts or greater) cannot produce pulses with high peak powers.
High-peak-power pulses can be produced by Q switching a laser. In Q switching, the "quality" or "Q" of the laser cavity is changed in order to generate a pulse. The size of conventional Q-switched lasers, along with the physics of the device, precludes the production of extremely short pulses.
Extremely short, high-peak-power pulses can be obtained from either Q-switched mode-locked lasers or amplified mode-locked lasers. Both of these approaches require large (typically several feet long), complicated (requiring daily supervision by a qualified laser technician), power-hungry (several kilowatts of electrical power), and therefore expensive devices.
It has recently been shown that coupled-cavity Q-switched microlasers can produce pulses of less than 300 ps duration with peak powers in excess of 25 kW.
Zayhowski, J. J. and Dill III, C., "Diode-Pumped Microchip Lasers Electro-Optically Q-Switched at High Pulse Repetition Rates," Optics Letters, Vol. 17, No. 17, 1201-1203, (Apr. 23, 1992). PA1 Zhou, S., et al., "Monolithic Self-Q-Switched Cr,Nd:YAG Laser", Optics Letters, Vol 18, No. 7, 511-512, (Apr. 1, 1993).
Thus, picosecond Q-switched microlasers can produce output pulses as short as large mode-locked lasers With peak powers as high as commercially available Q-switched systems. And, the entire device can fit into a package approximately the size of a standard diode-laser package with the possibility of battery-powered operation.
While coupled-cavity Q-switched microlasers outperform larger conventional devices in every way except average power, there is still room for improvement. In order to obtain proper Q-switching of the coupled-cavity microlaser, high-speed high-voltage electronics are required. The size, performance, and power consumption of the electronics limit the size, performance, and power efficiency of the coupled-cavity Q-switched microlaser system. In addition, the performance of the coupled-cavity laser relies on maintaining interferometric control of the relative lengths of the two constituent cavities, placing tight tolerances on the manufacture of the device and on the temperature control of the device during use.
The passively Q-switched microlaser does not require switching electronics, thereby reducing the size and complexity of the total system, and improving the power efficiency. In addition, there is no need for interferometric control of cavity dimensions, simplifying production of the device and greatly relaxing the tolerances on the temperature control of the device during use. The result is a potentially less expensive, smaller, more robust, and more reliable Q-switched system with performance comparable to that of coupled-cavity Q-switched microlasers. With this combination of attributes, passively Q-switched picosecond microlasers are very attractive for a large range of applications including micromachining, microsurgery, high-precision ranging, robotic vision, automated production, environmental monitoring, ionization spectroscopy, and nonlinear frequency generation.
In the current state-of-the-art, passively Q-switched lasers typically have a pulse length of tens of nanoseconds, although recently pulses of 3.5-ns duration have been demonstrated using a miniature laser constructed from a gain medium which simultaneously acts as a saturable absorber, as described in